Presses which employ a concave shoe engaged against a backing roll, such as the Extended Nip.RTM. press manufactured by Beloit Corporation of Beloit, Wis., are used in a papermaking machine for increasing the residence time of a web during passage of the web through a pressing nip. Typically, a nip is defined as the narrow region about the line of co-tangency when two rolls with aligned axes are brought together. The nip between rolls has classically been used in papermaking to remove water from the web, and to compress the fibers in the web into a smooth surface. In the papermaking art, it has been found to be desirable to increase the length of the nip, resulting in a somewhat lower pressure and uniform pressing, and increased effectiveness of pressing of the paper web. Initially, the length of the nip was increased by coating one or both of the rolls with a compliant material. The use of a compliant roll system allowed for nips somewhat greater than an inch in length along the direction of the web through the nip.
The desire for even longer nips led to the development of the so-called Extended Nip.RTM. press, or ENP. The ENP employs a backing roll and an elongated shoe, where the elongated shoe has a concave surface with a radius of curvature slightly larger than that of the convex surface of the backing roll. The shoe, which is typically made of steel, cooperates with the backing roll over a nip length of approximately ten inches along the direction of travel of the paper web.
In this configuration, the shoe is stationary. Thus, without further modification, a paper web moving through the nip formed between the shoe and the backing roll would experience unacceptable rubbing on the stationary shoe. To overcome this problem, a bearing blanket forming a cylindrical tube is slidably disposed over the shoe and around the support shaft on which the shoe is mounted. This bearing blanket supports the paper web as it passes through the nip. To reduce friction forces between the bearing blanket and the shoe, lubricant is supplied between the bearing blanket and the shoe, allowing the bearing blanket to slide freely across the shoe on a film of lubricant. A shoe of this type is known as a hydrodynamic shoe.
The type of lubricant used affects the thickness of the film layer. Higher viscosity lubricants create thicker film layers than do low viscosity lubricants; however, increased horsepower is needed to drive the backing roll. In addition, as the rotational speed of the backing roll decreases, the thickness of the film layer decreases. Thus, where the backing roll is operating at low speed, it is difficult to maintain the film layer.
To aid in drying or pressing of the paper web, a felt, or web support blanket, often underlies and supports the paper web as it transits the nip between the backing roll and the bearing blanket on the shoe. The backing rolls, paper web, and web support blanket (if present) are frictionally engaged, and in turn engage the upper surface of the bearing blanket, causing the bearing blanket to slide over the shoe and rotate about the shoe and its support shaft.
A typical papermaking machine of which the Extended Nip.RTM. Press forms a part can produce over one-half million square feet of paper per hour of operation. Thus, it is desirable that the machine be in operation as close to continuously as possible, with downtime for repair or replacement of parts being kept to a bare minimum. One part that is subject to replacement is the bearing blanket. As stated above, it is subject to friction forces as it rotates about the shoe and the shoe support shaft. Due to the large pressure force in the nip, the bearing blanket can also be damaged by paper wads or other deformation in the paper web that cause the lubricant film layer between the shoe and blanket to collapse. Blankets cost from about $100,000 to $200,000 each, so blanket damage from paper wads or other web deformities is a serious problem in the papermaking industry. Further, it takes from four to eight hours to replace a blanket, during which time no paper can be made.
In an attempt to minimize such damage to the bearing blanket, various shoe configurations have been proposed. One such configuration is known as a hydrostatic shoe. In a hydrostatic shoe, the concave portion of the shoe defines a pocket, so that at least for a portion of the travel of the bearing blanket through the nip, the blanket is hydrostatically supported with lubricant within the pocket. For example, in U.S. Pat. No. 5,262,011 to Ilmarinen ("the '011 patent"), pockets are disclosed having several pocket zones where the trailing edge of the pocket decreases to zero depth. Lubricant is supplied directly to the pocket by channel means passing through the shoe.
One of the objectives of the '011 patent is to provide a shoe configuration whereby damage to the bearing blanket from a paper wad entering the nip is reduced. However, if the hydrostatic pocket abruptly terminates at the trailing end thereof, there is a tendency for a paper wad to cause a sudden pressure surge as the wad moves between the blanket and the concave surface at the trailing end of the pocket, thus collapsing the lubricant film layer.
U.S. Pat. No. 5,441,604 to Sandberg et al. ("the '604 patent") also discloses a hydrostatic shoe, where the concave surface encompasses a pocket. In the shoe of the '604 patent, lubricant is supplied onto the concave surface upstream from the pocket and flows into the pocket. The pocket acts to relieve pressure on the bearing blanket caused by a paper wad entering the nip.
Both hydrodynamic shoes and hydrostatic shoes have rigid steel concave surfaces.
As a result, tight tolerances are required to maintain a sufficient lubricant film between the shoe and the bearing blanket. This is difficult and expensive to accomplish, due to the large surface area of the shoe. Moreover, hydrostatic shoes are expensive to manufacture, because the pockets therein must be machined to exact dimensions.
Thus, there exists a need for an elongate shoe that is easier and less expensive to manufacture, and that does not require tight tolerances. There also exists a need for an elongate shoe which can increase bearing blanket life, by significantly reducing damage to the blanket from paper wads and web deformities passing through the nip.